Will not start

First we have to get it cranking, otherwise nothing else matters. Check the battery cable connections first. Next, turn on the head lights or interior lights, then watch what happens to their brightness when you try to crank the engine.

Do you have a digital voltmeter and know how to use it? For some of the tests, you can get by with a test light. Either way this will be easier with a helper to turn the ignition switch. If you do not have a test light or voltmeter, you can find both at Harbor Freight Tools at very low cost. Their little red voltmeters cost around $8.00 and work just fine. I can tell you how to use it if necessary.

Clicking indicates either a low/bad battery or bad connections. So first you test the battery to be sure it is not the problem. Use the volt meter to test the battery, you should see 12.6 volts or a bit higher on a fully charged battery. Voltage any lower than about 11 means put a charger on it before you try to start it.
So you test it and find 12.2 volts, well that should crank the engine over but you just get clicks. Check the battery cables at both ends and wiggle the cables around. Many times a cable will corrode inside the actual connectors or even inside the rubber insulation. They look good visually but are actually not capable of carrying the amperage needed to turn the starter motor over. However, they may show a good voltage reading on the meter because one strand of wire still makes a connection.
Bad connections are very common on vehicles that set as the engine bay does not get hot which means any moisture that condenses on the parts gets to wick into areas it normally would not have the opportunity to. Rust or lead oxide are both poor conductors and it does not take very much to stop an engine from starting.

Buzzing is low amperage to the starter solenoid 99% of the time. Still the same issues apply. I say amperage because you can measure twelve volts with a meter but not have enough amperage to turn the starter motor over.

Is the power drop at the starter connections or are you seeing that at the battery end? Could be a bad starter but doubtful. Connections can look good but be bad inside the cable.

To go to 0 volts, depending on where you're taking that measurement, ( battery posts vs. Battery cable clamps), you either have a very bad battery or loose or corroded cables or connections. Even a locked-up starter won't draw a good battery down to 0 volts. You would have around 250 to 300 amps, which is the normal current draw for a starter motor not yet up to full speed. That normally lasts for a fraction of a second, then current drops to less than 150 amps for the remainder of the crank cycle. If the starter is locked up, which typically only happens when the engine is seized, that higher current will be constant and the battery cables will get real hot in a short time.

The clue to this is to put your meter probes right on the center of the battery's posts to take the measurement while trying to crank the engine, not on the cable clamps. If you find voltage stays relatively high during the attempted cranking, move the probes to the cable clamps and try again. If the reading is substantially lower now, one of those two connections is loose or dirty.